The invention generally relates to electrode structures deployed in interior regions of the body. In a more specific sense, the invention relates to electrode structures deployable into the heart for diagnosis and treatment of cardiac conditions.
The treatment of cardiac arrhythmias requires electrodes capable of creating tissue lesions having a diversity of different geometries and characteristics, depending upon the particular physiology of the arrhythmia sought to be treated.
For example, a conventional 8 F diameter/4 mm long cardiac ablation electrode can transmit radio frequency energy to create lesions in myocardial tissue with a depth of about 0.5 cm and a width of about 10 mm, with a lesion volume of up to 0.2 cm3. These small and shallow lesions are desired in the sinus node for sinus node modifications, or along the AV groove for various accessory pathway ablations, or along the slow zone of the tricuspid isthmus for atrial flutter (AFL) or AV node slow or fast pathway ablations.
However, the elimination of ventricular tachycardia (VT) substrates is thought to require significantly larger and deeper lesions, with a penetration depth greater than 1.5 cm, a width of more than 2.0 cm, and a lesion volume of at least 1 cm3.
There also remains the need to create lesions having relatively large surface areas with shallow depths.
One proposed solution to the creation of diverse lesion characteristics is to use different forms of ablation energy. However, technologies surrounding microwave, laser, ultrasound, and chemical ablation are largely unproven for this purpose.
The use of active cooling in association with the transmission of DC or radio frequency ablation energy is known to force the tissue interface to lower temperature values. As a result, the hottest tissue temperature region is shifted deeper into the tissue, which, in turn, shifts the boundary of the tissue rendered nonviable by ablation deeper into the tissue. An electrode that is actively cooled can be used to transmit more ablation energy into the tissue, compared to the same electrode that is not actively cooled. However, control of active cooling is required to keep maximum tissue temperatures safely below about 100xc2x0 C., at which tissue desiccation or tissue boiling is known to occur.
Another proposed solution to the creation of larger lesions, either in surface area and/or depth, is the use of substantially larger electrodes than those commercially available. Yet, larger electrodes themselves pose problems of size and maneuverability, which weigh against a safe and easy introduction of large electrodes through a vein or artery into the heart.
A need exists for multi-purpose cardiac ablation electrodes that can selectively create lesions of different geometries and characteristics. Multi-purpose electrodes would possess the flexibility and maneuverability permitting safe and easy introduction into the heart. Once deployed inside the heart, these electrodes would possess the capability to emit energy sufficient to create, in a controlled fashion, either large and deep lesions, or small and shallow lesions, or large and shallow lesions, depending upon the therapy required.
The invention provides electrode assemblies and associated systems employing a nonporous wall having an exterior for contacting tissue. The exterior peripherally surrounds an interior area. The wall is essentially free of electrically conductive material. The wall is adapted to assume an expanded geometry having a first maximum diameter and a collapsed geometry having a second maximum diameter less than the first maximum diameter. The assemblies and systems include a lumen that conveys a medium containing ions into the interior area. An element free of physical contact with the wall couples the medium within the interior area to a source of electrical energy to enable ionic transport of electrical energy from the source through the medium to the wall for capacitive coupling to tissue contacting the exterior of the wall.
In a preferred embodiment, the capacitive coupling of the wall is expressed in the following relationship:
{square root over (RPATH2+L +XC2+L )} less than RTISSUE 
where:       R    PATH    =            K              S        E              ⁢          ρ      s      
and
K is a constant that depends upon geometry of the wall,
SE is surface area of the element, and
xcfx81S is resistivity of the medium containing ions, and
where:       X    C    =      1          2      ⁢      π      ⁢              xe2x80x83            ⁢      fC      
and
f is frequency of the electrical energy, and   C  =      ϵ    ⁢                  S        B            t      
where:
xcex5 is the dielectric constant of wall,
SB is the area of the interior area, and
t is thickness of the wall located between the medium containing ions and tissue, and
where RTISSUE is resistivity of tissue contacting the wall.
The invention also provides systems and methods for heating or ablating body tissue. The systems and methods provide a catheter tube having a distal end that carries an electrode of the type described above. The systems and methods electrically couple a source of radio frequency energy to the electrically conductive element within the electrode body and to a return electrode in contact with body tissue.
According to this aspect of the invention, the systems and methods guide the catheter tube into a body with the wall in the collapsed geometry and then cause the wall to assume the expanded geometry at least in part by conveying a medium containing ions into the interior area of the body. The systems and methods then ohmically heat or ablate body tissue by transmitting radio frequency energy to the electrically conductive element for ionic transport through the medium to the wall for capacitive coupling to tissue located between the return electrode and the electrode.